The TDM/TDMA principle is well known in radio systems or passive optical networks (PONs), where it is employed to permit transmission between a single basestation and a plurality of outstations. In the downstream (basestation to outstation) direction, the information (traffic) is broadcast to all outstations, but upstream it is transmitted in bursts, each of which must be timed to avoid mutual interference (overlap) so that at any time the basestation only receives data from one outstation.
In our co-pending GB Applications 931291 1.2 and 9312910.4 (D A Fisher 10-2-1 and D A Fisher 11-3) there are described time division multiple access frame alignment techniques for use in marshalling the transmission from newly connected outstations without interfering with existing traffic transmissions. The basic method of these applications comprises employing pseudo random sequences at a level below the noise sensitivity of the base station receiver (for normal traffic). These sequences can be detected using correlation and their phase is used to determine the loop delay to the new outstation. A sequence generator is required at the outstation, and a reference generator is required at the basestation for correlation process. This is an upstream marshalling technique. The present invention is concerned with protection of the basestation.